DESCRIPTION (Adapted from the Principal Investigator's Abstract): The major objective of this project is to analyze the requirements for the activation and maintenance of a virus-specific cytotoxic T cell (CTL)-mediated response in the neonatal host. CTL play a critical role in controlling viral infections and the ability of the host to develop a CTL response is critical to the outcome of disease. The high morbidity and mortality of newborns infected with viruses is associated with their inability to develop a protective immune response. Therefore studies aimed at inducing a protective and long-lasting immune response in newborns are of high validity and are pursued in this project. The specific aims are as follows: 1) To analyze the effect of dose of virus on the induction of a Th1-type vs Th2-type response in newborns. 2) To analyze the effect of dose of viral vaccine on the induction of Th1-type vs. Th2-type response in newborns. 3) To determine the life-span and the protective effect of memory CTL activated in newborn mice by viral vaccines. 4) To examine the role of CD4+ T cells and antigen in the maintenance of the CTL memory response.